Traditionally, silicon wafers containing a plurality of integrated circuits have been either manually inscribed or written by laser with serial numbers so that the wafers can be readily tracked during the many required fabrication process steps. When, however, the wafer becomes diced into many integrated circuit chips, the tracking of discrete chips through the final fabrication and inspection steps becomes nearly impossible, especially for bipolar arrays.
In an effort to alleviate this problem, an eight digit serial number has been written by laser on the back side of each bipolar chip, at the corners thereof, thereby restoring chip trackability. Referring to FIGS. 1a and 1b which show, respectively, the front and back sides of a diced integrated circuit or chip indicated generally at 10 which is square in shape and which has each side of a given length A, contains on the front side 12, a plurality of metal contact balls 14. The back side 16 of the chip 10 is provided with an eight digit laser-written serial number comprised of two digits each, as at 18, located at respective corners as shown.
These chips 10 are normally inserted into, transported in, and inspected from a plastic carrier or linear device bank (LDB) as shown in FIGS. 2 and 3. The linear device bank, indicated generally at 20, consists of a platelike tray, indicated generally at 22, which may be formed of plastic, and top cover 24 and bottom cover 26, sandwiching tray 22. The chips 10 are maintained nested within respective cavities, indicated generally at 28, which cavities are of square horizontal cross-section and defined by upper inclined sidewalls 32 and lower vertical sidewalls 30 which join the inclined sidewalls. Each chip 10 nests in a given cavity 28 and each linear device bank is manufactured and sized so as to hold a maximum of 120 chips. The vertically straight sidewalls 30 are of a given length and widthwise dimension C which is greater than the side dimensions A of the chip, thereby providing room for the chip to move within its cavity 28.
To use the linear device bank 20, the plastic tray 22 is oriented as shown in FIG. 3 such that the tapered side walls 32 face upwardly. The bottom cover 26 is slid under the plastic tray 22 and, with the top cover 24 removed, the chips 10 are inserted into the cavities 28, one chip per cavity, such that the contact balls 14 of each chip face upwardly. Lastly, the top cover 24 is slid into place and the linear device bank is complete.
The presence of the bottom and top covers 26 and 24, respectively, prevents the chips 10 from falling out of the linear device bank. As may be appreciated, the contact balls 14 are visible when the top face of the plastic carrier tray 22 is face up. Also the chips 10 are each captured in their respective cavities 28 by the straight sidewalls 30 partially defining those cavities 28.
In order to minimize handling and chance of breakage, chip serial numbers are read while the chips are nested in the linear device bank. In order to make the eight digit serial number visible, the linear device bank must be flipped over, thereby orienting the chips relative to their cavities 28 in the manner shown in FIGS. 4a and 4b. As seen in FIG. 4a, the metal contact balls 14 now rest on the linear device bank top cover 24.
As may be appreciated, for a carrier holding 120 chips, it is difficult to accurately and precisely register the chips 10 in their cavities 28 so that their serial numbers may be read. Reference to FIG. 4b illustrates what happens in an effort to accurately register a chip 10 in its cavity 28 so that its serial number is readable. (In order to successfully read a chip, the chip must be aligned accurately to a known position to within .+-.0.025 mm in each of two orthogonal axes.) When the chip is oriented as shown in FIG. 4b, the chip must bank against two adjacent tapered sidewalls 32a, 32b. When viewed through the top of the inverted linear device bank with the bottom cover 26 removed, FIG. 4b, and with an appreciation of the location of the straight side walls 30 of tray 22 relative to the tapered sidewalls 32, it can be readily seen that five of the eight digits are partially obscured by the presence of the straight side walls of the tray 22, at each cavity 28.
The requirement that each chip 10 be registered against two adjacent walls 32a, 32b of each cavity 28, is necessary to insure that the reading laser spot can be moved in a known manner to impinge upon each of the eight digits of the chip serial number. If the chip 10 is allowed to float freely within the cavity, i.e. FIG. 4a, since the chip dimension A is less than the cavity dimension C, registration cannot be guaranteed. Such orientation is possible by tilting the linear device bank against two adjacent walls in each cavity as, for instance, by the imposition of mechanical vibrations against the linear device bank. Irrespective of the nature of achieving the precise registration required, several digits of each chip serial number are obscured, preventing a full eight digit serial number from being properly read.
Additionally, when utilizing mechanical vibration or the like to shift a plurality of chips into precise alignment, impact of a chip against one or more of the references surfaces may be result in a rebound effect and, even if precise registration is initially achieved, it may be lost prior to termination of the optical reading step.
It is, therefore, a primary object of the present invention to provide an improved chip registration mechanism which insures the required precise registration of an integrated circuit chip relative to one or more registration surfaces, which eliminates flutter in movement of the chips while the chips are moved into proper and precise registration position, and which maintains such registration during the period of time when said chips are read.
It is a further object of the present invention to provide an improved chip registration mechanism which is employed within existing linear device banks supporting a plurality of chips in line and column fashion and in which all of the chips may be simultaneously moved into and maintained in registered position with respect to one or more registration surfaces of the linear device bank cavites individually housing such chips.